The present invention relates to a light source device capable of controlling a spread angle of a condensed luminous flux from a light source and of irradiating a rectangular region evenly with a high illuminance, and a projection type display apparatus which incorporates such light source device.
FIG. 10 illustrates a prior art arrangement for a projection type display apparatus including a pillar optical element and a light valve, as disclosed in U.S. Pat. No. 5,634,704. In this Figure, a numeral 2 represents a light valve of transmission type, 3 a projection lens, 4 a screen, 11 a lamp, 12 an elliptical mirror, 130 a rod integrator, 14 a relay lens, 15 a field lens and L.sub.1 a luminous flux emitted by the lamp 11 and condensed by the elliptical mirror 12.
Describing the operation of the prior art arrangement, the center of emission of the lamp 11 is disposed in the vicinity of a first focal point of the elliptical mirror 12, whereby luminous flux emitted by the lamp 11 is condensed at a second focal point after its reflection by the elliptical mirror 12. The rod integrator 130 has an incident end face which is disposed in the vicinity of the condensing point or the second focal point of the elliptical mirror 12, and as will be described later, after being subjected to the total internal reflection within the rod integrator 130, the luminous flux is emitted from its emitting end face 130B. The relay lens 14 is disposed so that an image formed at the emitting end face 130B is formed on an area T.sub.1 to be irradiated, such conjugate relationship being indicated by broken lines. Rays proceeding to the right, as viewed in FIG. 10, from the image of the light source which is formed on a surface S.sub.1 by the elliptical mirror 12 transmit through the relay lens 14 as shown in solid lines and form a secondary image of the light source on a surface S.sub.2 located short of the region to be irradiated, whereupon they become divergent. The field lens 15 has a focal position in the vicinity of the secondary image and is disposed close to the surface to be irradiated. The field lens 15 has the function of converting rays shown in solid lines into parallel irradiation light L.sub.4, which irradiates the region to be irradiated T.sub.1.
A picture is displayed in the surface of the light valve 2 of transmission type in response to an electrical signal from a drive circuit, which is not specifically shown. Luminous flux L5 which transmits through the light valve 2 also transmits through the projection lens 3, which provides projected light L6 forming an enlarged picture on the screen 4 to be viewed. As is known, where the projection lens 3 is not telecentric, a field lens (not shown) may be suitably disposed immediately behind the light valve 2 (or toward the projection lens 3) or immediately in front thereof (toward the lamp 11), thus enhancing the efficiency of incidence of the irradiation light L5 upon the projection lens 3.
FIG. 11 is a cross section illustrating the operation of the rod integrator 130 of FIG. 10. Converging light L.sub.1 is condensed at a condensing point 130C located in the vicinity of an incident end face 130F for impingement into the rod integrator 130. For the sake of brevity, only rays appearing in y-z plane of the coordinate system are shown. Incident rays are subject to the total internal reflection by lateral surfaces 130T and 130U or directly proceed to the right straightforward to reach the emitting end face 130B. Luminous flux subjected to the total internal reflection by the lateral surfaces 130T and 130U irradiates the emitting end face 130B in superimposed manner depending on combinations of surfaces where the total internal reflection occurs as if it represents rays emitted from virtual point sources C.sub.1, C.sub.2, C.sub.3, C.sub.4 and the like located in a plane which includes the incident end face 130F, with consequence that the emitting end face 130B is irradiated with a good uniformity thereacross. It is to be noted that the rod integrator 130 is in the form of a square pillar, and accordingly, the superimposed irradiation of the end face 130B also takes place by light which has undergone the total reflection by the remaining two lateral surfaces when considered in the x-z plane.
It is a feature of an irradiation optical system which uses a rod integrator to irradiate a light valve that an image on the emitting end face of the rod integrator is focused on the light valve by a relay optical system. Thus, when the rod integrator has a sufficient length, if the illuminance distribution is nonuniform on the incident side, an irradiating flux having an improved uniformity and a configuration substantially congruent to a desired area to be irradiated can be obtained on the emitting side and is led to the light valve for improving the efficiency of utilization of light from the light source. When combined with a strictly controlled imaging optical system, there can be realized an irradiation optical system which exhibits a reduced chromatic aberration and a very high transmission.
A light valve irradiation system incorporating a rod integrator will now be considered with reference to FIG. 12A, wherein an image of a light source is indicated at 101, a region to be irradiated at 201 and an irradiation optical system at 1001. The area of the image is designated by S.sub.1, a total solid angle of flux emitted from the image 101 by .OMEGA..sub.1, the area of the region to be irradiated by S.sub.2, and a total solid angle of flux which transmits through the region 201 by .OMEGA..sub.2. Assuming for the moment that the irradiation optical system 1001 involves no loss, it is known that the product of the area and the solid angle of flux remains constant across the irradiation optical system. Thus EQU S.sub.1 .times..OMEGA..sub.1 =S.sub.2 .times.Q.sub.2 (1)
FIG. 12A shows an instance where S.sub.1 &lt;S.sub.2, and thus it follows that .OMEGA..sub.1 &gt;.OMEGA..sub.2.
When a prior art rod integrator 130 in the form of a square pillar is used, it will be seen from FIG. 12B that the total solid angle .OMEGA..sub.1 of flux emitted from the rod integrator 130 is equal to the total solid angle .OMEGA..sub.1 emitted from the image 101. While the location of the incident surface on the rod integrator is shown spaced from the image 101, it should be understood that they are coincident. Note that S.sub.1 &gt;S.sub.1 '. In general, the capacity of the optical system and the limitation imposed on the light valve often result in that S.sub.1 '&lt;S.sub.2, and a smaller value of .OMEGA..sub.2 is favored frequently in consideration of the display performance of the light valve and demands from a color separation/synthesis optical system and a projection lens system. It will be seen from the equation (1) that a smaller value of .OMEGA..sub.2 may be achieved by increasing S.sub.2 in comparison to S.sub.1 '. However, S.sub.2 must be substantially commensurate with the light valve, and can hardly be changed. On the other hand, reducing S.sub.1 ' to increase the magnification is undesirable since it limits the efficiency of utilization of light from the light source. A improvement may be made in condensing light from the light source such that the area S.sub.1 of the image of the light source can be reduced while increasing the efficiency of utilization of light relative to S.sub.1 ', but this generally involves a side effect of increasing .OMEGA..sub.1. In sum, in order to reduce .OMEGA..sub.2, there is a need for some condensing improvement to be made prior to the irradiation optical system which reduces .OMEGA..sub.1. With the prior art rod integrator 130, the light from the light source is transmitted while retaining its angular distribution, and accordingly, S.sub.1 on the light source side had to be increased in order to reduce .OMEGA..sub.1 at the sacrifice of the efficiency of utilization of light.
On the other hand, the size of the light valve is preferably reduced to reduce the volume of the optical system or the entire display apparatus in which it is installed, thus representing a tendency toward a reduction in S.sub.2. An irradiation optical system 1001 which satisfies the relationship S.sub.1 '&gt;S.sub.2 is a reduction optical system, which represents a more stringent requirement on .OMEGA..sub.1 according to the equation (1). Accordingly, it will be seen that it is desirable to provide an irradiation optical system 1001 which represents an enlarging system together with an apparent reduction in Si and .OMEGA..sub.1 on the light source side in order to enable an efficient irradiation of the light valve under the condition that the area S.sub.2 is reduced and the solid angle .OMEGA..sub.2 which is sought to be obtained is smaller. However, the requirement of providing an apparent reduction in S.sub.1 and .OMEGA..sub.1 is impossible to achieve with the existing rod integrator 130. As a consequence, the only choice is to allow an increase in .OMEGA..sub.2 in order to maintain the efficiency of the optical system, which in turn causes drawbacks including an increased cost of the projection lens and a degradation in the contrast and the color reproducibility.